Heavy downpours and thunderstorms could cause flash flooding across parts of the UK as the unsettled weather continues. Forecasters say as much as 100mm (4in) of rain could fall in some places, with the potential for between 20 and 30mm in the space of an hour. The Met Office has issued yellow warnings for thunderstorms across much of Scotland, Northern Ireland, northern England and north Wales from the early hours of Friday until the end of Saturday. The Met Office spokesperson Grahame Madge said: “We do have a very unsettled weekend in prospect, with a lot of it covered by a thunderstorm and rain warnings. “We have got an area of low pressure which will be milling around over the UK, bringing rain to quite large part. So I think everywhere can expect to see quite heavy showers. “We know the footprint of where we think the heaviest rainfall will be, but it’s like a boiling saucepan: you will get bubbles coming up, and trying to pinpoint where the next bubble will be is virtually impossible.” Heavy rain may also affect areas outside the warning zone but is less likely to cause flash flooding. Areas including Glasgow, Plymouth and Belfast should expect temperatures in the low 20s on Saturday. Parts of southern England such as Bristol and London may reach 23C, roughly average for this time of year. The lowest temperatures are expected to be about 14C in the Shetland Isles and Northern Ireland. Forecasters predict the weather will remain unsettled in in to next week with the chance of one-off calmer days.